I've used this app for years, and it helped me become fluent while living in Japan. It has a dictionary for both words and characters. It has sections divided which you can study, whether you're doing JLPT or kanji kentei, school years, or just a random topic you want to brush up on there's probably a section for you. The only thing I wish this app had was a searchable dictionary of sentence examples, like the Imiwa app has.